This invention relates to an apparatus for manufacturing embossed foil for tape, such as Nb.sub.3 Sn tape, suitable for use in superconducting magnets.
As is well known, a magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnetic coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible coil resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter called "MRI").
Considerable research and development efforts have been directed at eliminating the need for a boiling cryogen, such as liquid helium, and in providing superconducting magnets which maintain the magnetic state and do not "quench," or discontinue superconductivity. However, the differential thermal expansion and contraction between materials in superconducting magnets, which are cycled from ambient temperature to temperatures in the range of absolute zero (-270.degree. C.), and the extremely large magnetic forces provided, and utilized in a MRI lead to conflicting properties required of the materials, including the superconducting tapes used. In addition, the desired layered superconducting magnet materials for superconducting tapes such as Nb.sub.3 Sn are often relatively brittle and difficult to fabricate, and to handle, in winding coils.
Improved materials for use in fabricating superconducting magnets and associated superconducting tapes are disclosed in the copending United States patent application of Bu-Xin Xu, R. Jaykumar and John Wollan, entitled "Persistent Superconducting Switch for Conduction-Cooled Superconducting Magnet," Ser. No. 08/001,692 filed Jan. 7, 1993; and the copending patent application of Bu-Xin Xu and John Wollan, entitled "Stable Flux-Jumping Resistant Superconducting Tape and Superconducting Magnet," application Ser. No. 07/984,819, filed Dec. 3, 1992, both of which are assigned to the same assignee as the present invention and are hereby incorporated by reference. The subject invention is suitable for use in manufacturing superconducting tapes of the types disclosed in the aforementioned copending patent applications.
However, the manufacture of multilayer tape, such as Nb.sub.3 Sn tape, suitable for superconducting use in MRI involves passing long lengths of NbZr through a molten tin bath. A persistent problem encountered in such a process is controlling the rate and uniformity of the tin flow on the surface of the NbZr foil during the reaction sequence in forming Nb.sub.3 Sn. Patterning or embossing the NbZr foil such as by knurling can provide a surface foil topography which can better accept and control the flow and plating of the liquid tin in a tin bath.
In developing suitable equipment to form an embossed pattern on the foil by passing the foil through a knurling roll, a plurality of problems were encountered. The use of a knurling roll pressing on the foil, when backed by a compliant, but hard rubber roller resulted in the foil tending to "cup up" rather than lay flat. Another problem encountered is controlling the travel of the foil through the knurl rolls, that is steering the foil without it wandering in a lateral direction axial to the axis of the knurling roll. The tearing of the extremely thin NbZr foil, 0.001 inches thick, and problems in obtaining uniform depth of the embossed pattern were also encountered along with problems created by stress or cracks in the foil which can degrade the critical current carrying capability of the superconducting tape.